I never thought about never as having so many nuances in meaning, but I suppose it’s never too late to consider them.

Clearly, never in a pure semantic sense means not ever, suggesting a seemingly endless period of time. For example:

God never sleeps.”

“We never have made unsecured loans and likely never will.”

Then there seems to be a middle ground in sentences like “I never lied to you” in response to “You lied to me.” The person making the accusation is probably referring to a specific instance, whereas the respondent could easily mean never in the sense of not ever.

"I never heard you come in just now" and "You never mentioned that when I asked you the first time" both make reference to a specific occurrence and would be better as “I didn’t hear you come in….” and “You didn’t mentioned that….”

I wouldn’t call these sentences slang, however. Rather, I see them as examples of spoken English, where rules and pure semantics are often overlooked. That doesn’t make them ungrammatical. It just makes them informal.

I’m not sure that there is a more grammatical “thought” behind these less formal uses of never, but I don’t consider them slang or ungrammatical. I simply think that we don’t have the time to parse spoken English as we utter it.

My opinion is that all of your examples are good spoken English that might be written a bit differently.